In X-ray imaging, for example, in computed tomography, angiography or radiography, counting directly-converting X-ray detectors or integrating indirectly-converting X-ray detectors can be used.
In indirectly-converting X-ray detectors, X-ray radiation or photons can be converted into light by a suitable converter material and into electric pulses via photodiodes. Scintillators, for example, GOS (Gd2O2S), CsJ, YGO or LuTAG, are frequently used as converter material. Scintillators are particularly used in the energy range up to 1 MeV in medical X-ray imaging. Usually, so-called indirectly-converting X-ray detectors, so-called scintillator detectors, are used, in which the conversion of the X-rays or gamma rays into electrical signals takes place in two stages. In a first stage, the X-ray or gamma quanta are absorbed in a scintillator element and converted into optically visible light; this effect is called luminescence. In a second stage, the light induced by luminescence is then converted into an electrical signal by a first photodiode optically coupled to the scintillator element, read out by way of evaluation or readout electronics and then forwarded to an arithmetic unit.
In directly-converting X-ray detectors, the X-ray radiation or the photons can be converted into electric pulses by a suitable converter material. A converter element comprises the converter material. CdTe, CZT, CdZnTeSe, CdTeSe, CdMnTe, InP, TlBr2, HgI2, GaAs or others can be used as converter material, for example. A voltage is applied to the converter element such that the electron-hole pairs triggered by the X-ray radiation and/or photons can be separated. The electrical charge of the electrons or holes is forwarded to the evaluation electronics as an electric pulse. The electric pulses are assessed by the evaluation electronics, for example an integrated circuit (Application Specific Integrated Circuit, ASIC). In counting X-ray detectors, incident X-ray radiation is measured by counting the electric pulses which are triggered by the absorption of X-ray photons in the converter material. As a rule, the level of the electric pulse is proportional to the energy of the absorbed X-ray photon. As a result, spectral information can be extracted by comparing the level of the electric pulse with a threshold value.
The converter material can have drift characteristics which have an impact as a drift effect and/or change in the count values of the evaluation electronics and can thus produce unwanted image artifacts. These drift characteristics can be improved by way of additional illumination. The converter material can be, for example, conditioned, i.e. a predetermined condition produced, by way of the additional illumination. As a result of this conditioning, imperfections in the converter material can be reduced accordingly.
An X-ray detector is known from the publication DE 10 2010 015 422 A1 comprising a directly-converting semiconductor layer for the conversion of incoming radiation into electrical signals with a band gap energy characteristic of the semiconductor layer and at least one light source for coupling of light into the semiconductor layer, wherein the generated light has an energy above the band gap energy of the semiconductor layer for the simulation of incoming X-ray quanta. Furthermore, it comprises at least one evaluation unit for calculating an evaluation signal from the electrical signals generated when coupling the light into the semiconductor layer and at least one calibration unit for calibrating at least one pulse discriminator based on the evaluation signal. This will create the preconditions for a rapidly repeatable calibration of the X-ray detector, taking into consideration the current polarization state without the use of X-ray radiation.
A directly-converting X-ray radiation detector, particularly for use in CT systems, is known from the publication DE 10 2011 080 892 B, having at least one semiconductor material used for the detection of X-ray radiation. A scintillation layer is applied to at least one of the sides of the semiconductor material facing the X-ray radiation, wherein the X-ray radiation generates optical radiation in the scintillation layer.
A directly-converting X-ray detector for the detection of X-ray radiation is known from the publication DE 10 2013 214 684 A1, having a direct converter used for the detection of X-ray radiation, at least one collimator arranged at least partially in the direction of radiation of the X-ray radiation before the direct converter and at least one radiation source which is arranged laterally of the direct converter and indirectly irradiates the direct converter with additional radiation, wherein the at least one collimator has at least one reflexive coating on a side facing the direct converter, on which the additional radiation is reflected onto the direct converter, and having a cooling device by which the at least one radiation source can be cooled.
The radiation source and/or light source was hitherto soldered onto for example printed circuit board material, which was mounted on the collimator and/or anti-scatter grid via clamping or bonding laterally between the side of the anti-scatter grid facing the converter element and the plane of the surface of the converter element facing the anti-scatter grid.